Current meter or flow meter

ABSTRACT

A flow meter in which a cylindrical device is immersed in the fluid stream and produces Karman vortices and in which tubes pass through the cylindrical device and supply a quantity of fluid which varies as a function of the pressure on the surface of the cylindrical device and wherein a velocity measuring means is mounted in one of the tubes to detect the flow through the tube.

United States Patent Tomota et al.

CURRENT METER OR FLOW METER [56] References Cited lnVemOlSI MiyqiiTomota; Yutaka lshikawa; UNITED STATES PATENTS Biron Yamasaki; YoshioKurita, all of Tokyo, Japan 2,813,424 11/1957 Liepmann et a1. ..73/l94 83,116,639 1/1964 Bird ..73/194 8 Assignee: Kabushikiklisha YokogawaDenkl 3,564,915 2/1971 Tomota et a] ..73/ 194 C Seisakusho (YokogawaElectric 3,422,682 1/ 1969 Evans et a]. ..73l212 Japan OTHERPUBLICATIONS Filed: Sharpsteen, Fluid Amplifier Measures Flow AppL No:52,967 Velocity ,Control Engineering, Jan. 1966. p. 103.

Primary Examiner-Charles A. Ruehl Foreign Amman" Priomy DimAttorney-Hill, Sherman, Mei-om, Gross 81. Simpson July 18, 1969 Japan..45/56885 [57] ABSTRACT Jan. 30, 1970 Japan A flaw meter i which a c d1 y m "C device 18 im- .Ian. 30, 1970 Japan ..45/ 8197 mersed in hefluid stream and produces Karma [970 Japan tices and in which tubes passthrough the cyiindricai device and supply a quantity of fluid whichvaries as a UOSO 1 s s s n u v q 1 n q u s s s e n Q u n I v s n B Int.cal device and wherein a velocity measuring means is Field 01 Search..73/194 B, 194 C, 212 mounted i one f the tubes to detect the n throughthe tube. 1

15 Claims, 26 Drawing Figures PATENTEU SEP I 9 m2 3.691. 830

sum 3 or 9 PAIENTED E I 9 9 3.691.830

saw u or 9 X 32 3/ J 3 A 35 o g 37 34 f q 33 34 33 YL32 C3 PATENTEDSEP 9m2 SHEU 8 BF 9 fi- -HA FT-BB PATENTEDSEP 19 I972 3.691. 830

T/l/l/l/ [III F (\Io) Pdfo) I I I I I I I I I I II PAIENTED E 9 \93.691. 830

sum 9 or 9 jiffy-J1 CURRENT METER OR FLOW METER BACKGROUND OF THEINVENTION It is well known in the art that when such an object 0 I, forexample, a cylinder with circular cross-section is immersed in a fluidstream F as shown in FIG. 1, that inward-spinning vortices e aregenerated alternately on both sides behind the object I at regularintervals and alternately, which vortices e shed or separate from theobject 1 in two staggered-parallel rows and are carried downstream.These asymmetric rows of the vortices e are commonly referred to as theKarman vortex street in the art and is a phenomenon which has long beenstudied. The growth and shedding cycle of the wake flow patterncomprises a staggered trail of vortices.

It is also well known in the art that these vortices are shed downstreamat a frequency determined by the flow velocity or flow rate.

The number of Karman vortices formed per unit time is designated f andis expressed by the following equation.

where V represents the flow velocity of the fluid F D the diameter ofthe cylindrical object 1' and K is a constant.

It will be, therefore, apparent that the flow velocity of the fluid orthe quantity of flow (flow rate) can be obtained by measuring f.

SUMMARY OF THE INVENTION One object of the present invention is toprovide a current meter or flow meter which can accurately measure thenumber of Karman vortices formed even if the fluid is contaminated,contains dusts or solid particles.

Another object of the present invention is to provide a current meter orflow meter in which the detecting elements may be easily repaired orreplaced.

Still another object of the present invention is to provide a simple andcompact current or flow meter.

Other objects, features and advantages of the present invention willbecome apparent from the following description taken in conjunction withthe accompanying drawings, wherein like reference numerals throughoutthe various views of the drawings are intended to designate similarelements or components.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows formation of Karmanvortices;

FIGS. 2A and 2B are schematic diagrams showing one example of thepresent invention;

FIG. 3A is a side view of the cylindrical object employed in the exampleof the present invention depicted in FIGS. 2A and 28',

FIG. 3B is a cross-sectional view along the line X-X in FIG. 3A;

FIG. 3C is a cross-sectional view along the line Y-Y in FIG. 33;

FIG. 4 is a schematic diagram for explaining the operation of theembodiment shown in FIG. 2;

FIG. 5 is a schematic diagram illustrating another example of thepresent invention;

FIG. 6A is a perspective view illustrating another modification of thecylindrical object usable in the present invention;

FIG. 6B is a cross-sectional view along the line X-X in FIG. 6A;

FIG. 6C is a cross-sectional view along the line Y-Y in FIG. 68;

FIGS. 7A, 7B, 7C and 8A, 8B, 8C respectively show further modificationsof the cylindrical object usable in the present invention similar tothat of FIGS. 6A, 6B and 6C;

FIG. 9A shows a further modification of the present invention partly cutaway;

FIG. 9B is a cross-sectional view along the line X-X in FIG. 9A;

FIG. 9C is a cross-sectional view along the line Y-Y in FIG. 9B;

FIG. 10A shows a still further example of the present invention partlycut away;

FIG. 10B is a cross-sectional view along the line X X in FIG. 10A;

FIG. 10C is a cross-sectional view along the line Y Y in FIG. 10B; and

FIGS. 11 to 13, inclusive are cross-sectional views for illustratingmodifications of the cylindrical object usable in the present inventionsimilar to that, for example, FIG. 103.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 2A and 2B show oneembodiment of the present invention. Reference numeral 1 indicates acylindrical object for producing the Karman vortex. In this example theobject 1 has a circular cross-section. Reference numerals 21 and 22 arepipes or tubes of small diameter which have end portions that extendinto the cylindrical object 1 and open on opposite sides thereof at 31,32. These openings or ports 31, 32 of the pipes 21, 22 are used tosupply fluid from reservoir 5 through the pipes 21, 22 in accordancewith pressure variation of the fluid F near the openings 31, 32. Thesepressure variations are caused by the vortices produced. The pipes 21,22 are used for supplying a fluid for purging or cleaning the surface ofthe cylindrical object 1. Reference numerals 41, 42 designaterestrictors mounted in the pipes 21, 22, respectively. A purge fluidsource S is connected to the other ends of the pipes 21, 22 and suppliespurge fluid to the pipes 21, 22 at a substantially constant pressure. Afluid displacement detecting element 6 such, for example, as a hot wireis constructed so as not to obstruct the displacement of the fluid andis mounted in the pipe 21 to detect the variations of the velocity orquantity of the flow of the fluid.

FIGS. 3A, 3B and 3C illustrate the cylindrical object 1 depicted inFIGS. 2A, 28 on an enlarged scale. As shown in these figures the pipes21, 22 through which the purge fluid flows are introduced into thecylindrical object 1 and open at ports 31, 32 on opposite surfaces ofthe cylindrical object 1 at approximately its midpoint. The openings 31,32 open on an axis at right angles to the fluid stream, so that thepurge fluid supplied from the purge fluid source 5 purges or cleans thesurface of the cylindrical object 1 near the openings.

When the cylindrical object 1 is immersed in the fluid stream F flowingthrough a conduit in such a manner that the longitudinal axis of thecylindrical object 1 and the line connecting the opennings 31 and 32 areboth perpendicular to the flow direction of the fluid F, vortices willbe alternately produced behind the cylindrical object 1 at regularintervals based upon the theory of formation of the vortex describedabove. When the vortex is produced the flow velocity of the fluid alongthe surface of the cylindrical object 1 changes and the pressures of thefluid on opposite sides of the cylindrical object 1 vary in accordancewith the formation of the vortices and their separation from thecylindrical object 1. The purge fluid from the purge fluid source 5passes through the pipes 21, 22 and out of the openings 31, 32 to purgeor clean the surface of the cylindrical object 1. The quantity of theflow of the purge fluid which flows out of the openings 31, 32 changeswith the pressure of the fluid F near the openings 31 32. As shown inFIG. 4, immediately after a vortex e is produced near the opening 31 anda vortex separates from the cylindrical object 1 near the other opening32, the velocity of the fluid flowing along the surface of thecylindrical object 1 becomes higher on the side of the cylindricalobject 1 near opening 32, and the pressure of the fluid F on the surfaceof the cylindrical object 1 in the vicinity of the opening 32 becomessmaller which increases the amount of purge fluid supplied from theopening 32. On the other hand, the flow velocity of the fluid F on theside of the cylindrical object 1 adjacent the opening 31 becomes lowerand the pressure of the fluid F exerted on the cylindrical object 1 inthe vicinity of the opening 31 becomes higher which decreases the amountof purge fluid supplied from the opening 31. Thus, the amount of thepurge fluid supplied from openings 31 and 32 varies in accordance withthe number of vortices produced on the opposite sides of the cylindricalobject 1. The variation in the quantity of the purge fluid jetted fromthe openings 31, 32 causes variations in the flow velocity of the purgefluid past the restrictors 41, 42 mounted in pipes 21, 22 and thus thenumber of vortices produced near the opening 37 can be measured bydetecting the variation of the velocity or quantity of the purge fluidflowing through the pipe 21, with, for example, a hot wire 6. Theelectrical signal obtained from the element 6 is proportional to thevariations of the velocity or quantity of the purge fluid and issupplied to a suitable meter (not shown).

With the above arrangement because the purge fluid is always ejectedfrom both sides of the cylindrical object 1, solid particles and/or dustwill not adhere to the surface of the cylindrical object and theformation, growth and separation of the Karman vortices will be assured.Further, the fluid displacement detecting element 6 is disposed in thepipe 21 and detects the variations of the velocity or quantity of theflow of the purge fluid which is not contaminated and the element 6 isnot directly in contact with the fluid F whose velocity is to bemeasured. Thus, even if the fluid to be measured is a contaminated gas,the number of vortices can still be accurately detected.

Also, since the detecting element 6 is located in the portion of thepipe outside of the cylindrical object 1, it is very easy to repair orexchange it.

FIG. 5 shows a modified form of the present invention. The samereference numerals as used in the foregoing examples designate likeelements. Reference numeral 1 indicates the cylindrical object forproducing the Karman vortex, 21 and 22 indicate the pipes or tubes ofsmall diameter which supply the purge fluid, a plurality of openings 31,32 are formed in longitudinal rows in tubes 21 and 22 and open on theopposite sides of the cylindrical object 1; 41 and 42 are restrictorsmounted in tubes 21 and 22; 5 is the purge fluid source for feedingpurge fluid of substantially constant pressure to the tubes 21, 22; and,6 and 6' are fluid displacement detecting elements mounted in tubes 21,22 for detecting the variations of the velocity or quantity of the purgefluid in tubes 21, 22. Reference numerals 63, 64 are resistors connectedin series and 65 is a DC power source inserted between the free ends ofthe resistors 63 and 64. The elements 6 and 6' are connected in seriesand their free ends are respectively connected to opposite terminals ofthe DC power source 65, whereby the elements 6, 6' and the resistors 63,64 form a bridge circuit. The DC power source 65 is connected across thebridge and the amplifier 7 is connected across the bridge. The amplifieris connected to output terminal 8.

In FIG. 5 the plurality of openings 31 and 32, open on the oppositesides of the cylindrical object ii along its axial direction and thepurge fluid from the purge source 5 is ejected uniformly from thecylindrical object 1 along the axial direction of the member 1. As aresult dust or solid particles are positively and effectively preventedfrom sticking on the surface of the cylindrical object 1.

Also, the velocity or quantity of the purge fluid flowing through thetubes 21, 22 varies with the variations of the pressure of fluid Fcaused by the vortices alternately produced on both sides of thecylindrical object l and output signals indicative of the number ofvortices formed can be obtained at the output terminals of the bridgecircuit because the resistance values of the elements 6 and 6' vary inaccordance with the variations of the velocity or quantity of the flowof the purge fluid flowing through the tubes 21, 22.

F165. 6A, 6B and 6C show a modified form of the cylindrical object. Inthese figures reference numerals similar to those in the foregoingexamples indicate similar elements. In FIGS. 6A to 6C, the openings 31,32 of the tubes 21, 22 terminate in cavities 33, 34 formed in thecylindrical object 1. Reference numerals 35 and 36, respectively,indicate a plurality of openings which are formed in the cylindricalobject 1 on opposite sides thereof in its axial direction. The openings35, ,36 intercommunicate with the cavities 33, 34 and introduce thefluid F into the cavities 33, 34 as a function of the pressurevariations of the fluid F caused by formation of the Karman vortices.Reference numeral 37 represents a partition wall provided in thecylindrical object 1 between the cavities 33 and 34 and an aperture 38is formed through the partition wall 37 so that the cavities communicatewith each other. One of the tubes 21 or 22 has mounted therein the purgefluid displacement detecting element 6.

If the cylindrical object 1 of FIGS. 6A to 6C is immersed in the fluid Fin such a manner that the longitudinal axis of the cylindrical object 1is substantially perpendicular to the flow direction of the fluid F anda line connecting the openings 35 and 36 is also approximatelyperpendicular to the flow direction of the fluid F, the fluid passinginto the cavities 33, 34 through the openings 35, 36 will flow throughthe openings 35, 36, the cavities 33, 34 and the aperture 38 inaccordance with the variations of the pressure of the fluid caused byformation of the vortices near the openings 35, 36 or their separationtherefrom. The purge fluid ejected from the openings 31, 32 of the tubes21, 22 into the cavities 33, 34 purges and cleans the cavities 33, 34and the openings 35, 36 and then is ejected to the outside of thecylindrical object 1. When the fluid in the cavities 33, 34 is displaceddue to the formation of the vortices or their separation, the fluidpressure in the cavities 33, 34 varies accordingly. Consequently thequantity of the purge fluid ejected from the openings 31, 32 of thetubes 21, 22 into the cavities 33, 34 varies according to the pressurevariations in the cavities 33, 34. Therefore, the number of vorticesformed can be obtained by detecting the variations of the quantity orvelocity of the flow of the purge fluid in the tube 21 with thedetecting element 6 disposed therein.

With this cylindrical object suction and emission of fluid occurs at theopenings 35, 36 in response to the formation of the vortices and/or itsseparation from the cylindrical object and the formation of the vorticescan be controlled by the suction and emission of the fluid. Thus, theformation of the vortices can be further stabilized so that fluctuationsof the fluid and vortices other than the Karman vortex in the fluid willnot introduce errors. ln general, when the boundary layer of the fluidalong the surface of an object with circular cross-section is ready toseparate from the surface of the object, such separation will beenhanced if fluid is ejected from the surface of the object near theseparation point, while the separation of the boundary layer is delayedif suction of fluid occurs near that point.

FIGS. 7A, 7B and 7C, show another modified form of the cylindricalobject 1. Similar reference numerals to those of FIGS. 6A, 6B and 6Crepresent similar elements. In this embodiment slits 35 and 36 are boredin the cylindrical object 1 on opposite sides thereof and along itsaxial direction. These slits correspond to the plurality of openings 35,36 in the FIG. 6 example. The openings 31, 32 of the ends of tubes 21,22 open into the slits 35, 36, respectively. The other construction issubstantially the same as that of HG. 6.

When the cylindrical object 1 shown in F165. 7A to 7C is immersed in thefluid F, the fluid received into the cavities 33, 34 through the slits35, 36 is displaced or flows through the slits 35, 36, the cavities 33,34 and the aperture 38 formed in the partition wall 37. The purge orclean fluid passing through the tubes 21, 22 is ejected from theopenings 31, 32 and purges and cleans the slits 35, 36 and passesoutside of the cylindrical object l. The velocity or quantity of theflow of the purge fluid ejected from the openings 31, 32 of the tubes21, 22 varies proportionally to the variations of the pressure of thefluid in the slits 35, 36 and thus to the number of Karman vorticesformed. Consequently, the number of Karman vortices formed can bemeasured by detecting the variations of the quantity or velocity of theflow of the fluid passing through the tubes 21 and 22 with fluiddisplacement detecting elements 6 and 6.

FIGS. 8A, 8B and 8C show another modified form of the cylindrical object1 in which like reference numerals designate elements corresponding tothose in the prior figures. In this example a single pipe 21 is employed for supplying cleaning fluid and its opening 31 opens intoaperture 38 of the partition wall 37. The other construction issubstantially the same as that of the FIG. 6 example.

When the cylindrical object I of FIGS. 8A to BC is immersed in the fluidF it flows into the cavities 33, 34 through the openings 35, 36 and isdisplaced or flows through the openings 35, 36, the cavities 33, 34 andthe aperture 38 formed in the partition wall 37 The purge fluid from thetube 21 is ejected from the opening 31 and purges the aperture 38, thecavities 33, 34 and the openings 35, 36 and passes to the outside of thecylindrical object 1. The quantity or velocity of the flow of the purgefluid ejected from the opening 31 of the tube 21 varies in response tothe pressure variations of the fluid in the aperture 38 whichcorresponds to the number of Karman vortices formed. Consequently, thenumber of vortices formed can be measured by detecting the variations ofthe quantity or flow velocity of the purge fluid flowing through thetube 21 with the detecting element 6.

FIGS. 9A, 9B and 9C show another example of the present invention inwhich reference numerals similar to those of the foregoing examplesindicate similar components. The arrangement is substantially the sameas that of FIG. 6 except that the fluid displacement detecting element 6is disposed in the aperture 38 and a static pressure detector 9 of thefluid F is provided in the fluid F flowing through the conduit 10. Theoutput from the static pressure detector 9 is applied to the purge fluidsource S to control it to maintain the pressure P, of the purge fluidsubstantially the same as the pressure P, of the fluid F to be measured.

With such an arrangement, if the fluid F has a density of p,,; and itsflow velocity v,,; and if the output pressure of the purge fluid is P,;its density is p,; and the quantity flowing through the tubes 21, 22 isQ; and that the mean pressure of the fluid in the cavities 33, 34 is P,then the following relationships exist:

oPtfi.' iF'iQ From the equation (3), the quantity Q of the purge fluidflowing through the tubes may be expressed as:

It will be apparent from equation (4) that the quantity 0 isproportional to the velocity v, of the fluid to be measured.

With the arrangement illustrated in FIGS. 9A to 9C the output pressureP, of the purge fluid from the purge fluid source 5 is controlled so asto be approximately the same as the static pressure P, of the fluid tobe measured and the restricters 41, 42 are provided in the tubes 21, 22respectively, so that the quantity of the purge fluid flowing throughthe tubes 21, 22 varies automatically with the variations of thevelocity v, of the flow of the fluid F to be measured. Consequently,even if the velocity of the flow of the fluid F to be measured varieswithin a comparatively wide range, the purge fluid can be effectivelyejected to the cavities 33, 34 and the openings 35, 36. When thevelocity v,, of the flow of the fluid F to be measured is comparativelylow, the energy in the vortices produced in the fluid by the cylindricalobject I is small and tends to be disturbed. In such case the quantityof flow of the purge fluid will be automatically decreased, so that thepurge fluid does not disturb the formation of vortices. In the casewhere the velocity v,, of the fluid F to be measured is rather high, thepressure difference between the opposite sides of the cylindrical objectbecomes great due to the formation or separation of the vortices andincreases the displacement or flow of the fluid F near the cavities 33,34 or the aperture 38. However, this causes the quantity 0 of the purgefluid to be automatically increased so that dust and/or particles areprevented from entering into cavities 33, 34 and from outside thecylindrical object 1. If they do enter they are repelled.

The variations of flow through the aperture 38 corresponds immediatelyto the pressure P of the fluid F in the cavities 33, 34 and the pressureP to the formation or separation of the vortices by the cylindricalobject 1, so that signals corresponding to the number of Karman vorticesformed can be obtained with the fluid displacement detecting element 6provided in the aperture 38.

FIGS. 10A, 10B and 10C illustrate another modified form of the presentinvention in which reference numerals similar to those of the foregoingexamples indicate similar components.

In this example the tubes 21, 22 which have mounted therein therestrictors 41, 42 are connected by a common portion which is opened tothe atmosphere. The fluid displacement detecting element 6 is mounted intube 21. The other constructional features are substantially the same asthat shown in FIGS. 9A to 9C except the detector 9 is not used. That is,in this example the purge fluid source which is used in the FIG. 9example is dispensed with and the atmosphere is used as the purge fluid.The element 6 is disposed in the tube 21 instead of being mounted in theaperture 38 of the FIG. 9 example.

The arrangement illustrated in FIGS. 10A to 10C can be employed withgood results where the static pressure of the fluid F to be measured islower than or about equal to that of the atmosphere.

If the static pressure of the fluid F to be measured is P,,; its densityis p; the velocity of the flow is v the pressure of the atmosphere is Pthe density of the atmosphere is m; the quantity of the flow of thepurge fluid flowing through the tubes is Q; and the mean pressure of thefluid in the cavities is P, the following equations (5), (6) areobtained:

nptt o IPIQ (71 Therefore, the quantity of the flow of the purge fluidflowing through the tubes is expressed as follows:

where K is a constant and is equal to It is apparent from equation (8)that the quantity 0 of the flow of the purge fluid is proportional tothe velocity v of the flow of the fluid F to be measured.

The operation and advantage attained by this example are substantiallythe same as those obtained by the example shown in FIGS. 9A to 9C, andthus will not be repeated.

The element 6 may be placed in the aperture 38 as in the case of theFIG. 9 example if desired.

FIGS. II to 13 respectively show other examples of the cylindricalobject I employed in the present invention in which reference numeralssimilar to those in the foregoing examples indicate similar components.

The embodiment of FIG. 11 has a groove or hole g formed in its partitionwall 37. The free end portion or opening of the single tube 21 whichpermits passage of the purge fluid ends in the groove or hole g, so thatpurge fluid flows through the hole g to the aperture 38 and purges thehole g and the bore 38. The fluid displacement detecting element 6 isdisposed in the aperture 38 in this example.

The modification shown in FIG. 12 is substantially the same as that ofFIG. 11 except that two fluid displacements detecting elements 6 and 6'are disposed in the aperture 38 instead of a single detecting element 6.

In FIG. 13, two tubes 2] and 22 which carry the purge fluid are employedand their ends are opened into two semi-circular grooves 31a and 320formed in the cylindrical object I. The free ends of each of thesemi-circular grooves 31a and 320 are opened at the openings 35 and 36respectively, so that the purge fluid emitted from the openings of thetubes 21, 22 purge the portions near the openings 35, 36.

The foregoing explanation relates to the case where one or both ends ofthe tube or tubes are open to the atmosphere or receive a specific purgefluid but it is also possible that the end or ends may be supplied purewater which may serve as the purge fluid. In such arrangement thespecific purge fluid source 5 can be dispensed with. Also, mean pressurein the cavities 33, 34 will be automatically decreased with an increaseof the flow velocity in the cavities, so that the quantity of flow ofthe purge fluid will be controlled by the velocity of the fluid to bemeasured and will always effectively purge and clean the cylindricalobject 1.

it is preferred to employ a hot wire, thermistor, resistor fortemperature measurement, or thermocouple, as the displacement detectingelement which has a temperature variation caused from cooling of theflowing fluid. It is not always necessary to employ an object withcircular cross-section as objects with elliptical or rectangularcross-sections can be used.

Since the purging and cleaning of the cylindrical object as well as thecavities, the openings, apertures, and the like can be accomplished withuncontaminated fluid and since the fluid displacement detecting elementis disposed in the tube through which the pure purge fluid flows, thenumber of Karman vortices formed can be accurately measured and with thesimple device even if the fluid to be measured is contaminated. Also,the fluid displacement detecting element can be easily repaired orexchanged if mounted outside the member.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts of thepresent invention.

We claim as our invention:

1. A flow meter comprising:

a cylindrical object immersed in fluid whose velocity is to be measuredand producing Karman vortices therein;

a purge fluid reservoir;

purge fluid conduit means connected to said reservoir and extending intosaid cylindrical object and terminating in openings on opposite sidesurfaces of said cylindrical object, said openings being formed to besubstantially perpendicular to the flow direction of said fluid, saidpurge fluid conduit means conducting purge fluid from said purge fluidreservoir to flow out of said openings of said cylindrical object, thepressure of said purge fluid in said reservoir being greater than thestatic pressure of said fluid surrounding said cylindrical object; and

velocity detecting means mounted in said purge fluid conduit means anddetecting the variations of velocity of said purge fluid in said purgefluid conduit means which is indicative of the flow velocity of saidfluid.

2. A flow meter as claimed in claim 1 wherein said purge fluid conduitmeans comprises a pair of tubes which extend into said cylindricalobject and are joined together and connected to said reservoir through acommon conduit and said velocity detecting means is mounted in one ofsaid tubes.

3. A flow meter as claimed in claim I wherein openings in saidcylindrical object are a plurality of openings on each side thereofaligned in its axial direction.

4. A flow meter as claimed in claim I wherein said openings in saidcylindrical object are a pair of slits on each side thereof andextending in the longitudinal direction of said cylindrical object.

5. A flow meter as claimed in claim 1 in which said purge fluid conduitmeans have restrictors mounted therein.

6. A flow meter as claimed in claim 1 in which said purge fluid is air.

7. A flow meter as claimed in claim 1 in which said purge fluid is aliquid and the pressure of said liquid is approximately equal toatmospheric pressure.

8. A flow meter comprising:

a cylindrical object to be immersed in fluid whose velocity is to bemeasured and producing Karman vortices in said fluid;

a purge fluid reservoir;

purge fluid in said reservoir;

at least two openings formed in said cylindrical object on oppositesides thereof and said openings formed to be substantially perpendicularto the flow direction of said fluid;

a pair of cavities formed in said cylindrical object on opposite sidesthereof and said two openings communicating therewith;

a partition wall between said cavities;

an aperture formed in said partition wall;

a pair of tubes connected to said reservoir and respectivelycommunicating with said cavities;

purge fluid supplied to said tubes which passes through said apertureand the pressure of said purge fluid in said reservoir being greaterthan the static pressure of said fluid surrounding said cylindricalobject; and

velocity detecting means for measuring the flow of said purge fluid.

9. A flow meter as claimed in claim 8 wherein said velocity detectingmeans is mounted in one of said tubes for detecting the variations ofvelocity of said purge fluid.

10. A flow meter as claimed in claim 8 wherein said velocity detectingmeans is mounted in said aperture.

1 l. A flow meter comprising:

a cylindrical object to be immersed in fluid whose velocity is to bemeasured and for producing Karman vortices in said fluid;

a purge fluid reservoir for supplying purge fluid to purge saidcylindrical object, the pressure of the purge fluid in said reservoirbeing greater than the static pressure of said fluid surrounding saidcylindrical object;

a pipe with one end connected to said purge fluid reservoir and feedingsaid purge fluid out of an opening in said cylindrical object to purgeat least one side surface of said cylindrical object and said openingformed to be substantially perpendicular to the flow direction of saidfluid; and

a fluid displacement detecting element mounted in said pipe means fordetecting the variations of velocity of said purge fluid.

12. A flow meter as claimed in claim 11 in which said cylindrical objecthas provided therein a plurality of axially aligned openings adjacentsaid openings on at least one side surface thereof and said plurality ofopenings formed to be substantially perpendicular to the flow directionof said fluid.

13. A flow meter as claimed in claim 12 wherein said opening in saidcylindrical object is a slit extending in the axial direction and formedto be substantially perpendicular to the flow direction of said fluid.

14. A flow meter as claimed in claim 11 in which said cylindrical objecthas formed therein two cavities, a partition wall between said cavities,an aperture formed in said partition wall, and said opening comprises apair or aperture so that it flows out of said groups of alignedopenings.

15. A flow meter as claimed in claim 14 in which said fluid dispiacementdetecting element is mounted in said aperture.

l l I I I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,691,830 Dated September 19, 1972 Inventor(s) Tomota et a1hove-identified patent It is certified that error appears in the a d asshown below:

and that said Letters Patent are hereby correcte Column 6, equation (3),rewrite to read as follows:

- Column 8, equation (5), rewrite to read as follows:

P Q S 2 0 P) K0 0 V0 Column 8, equation (8), rewrite to read as follows:

: K'VO Signed-and seeled this 9th day of July 1974.-

(SEAL) Attest:

c. MARSHALL DANN MCCOY M. GIBSON, JR. Attesting Officert Commissioner ofPatents USCOMM-DC 60376-9 FORM PC4050 (049D 0 u s eovuouunv "lunar.ornclt nn 0-.-

1. A flow meter comprising: a cylindrical object immersed in fluid whosevelocity is to be measured and producing Karman vortices therein; apurge fluid reservoir; purge fluid conduit means connected to saidreservoir and extending into said cylindrical object and terminating inopenings on opposite side surfaces of said cylindrical object, saidopenings being formed to be substantially perpendicular to the flowdirection of said fluid, said purge fluid conduit means conducting purgefluid from said purge fluid reservoir to flow out of said openings ofsaid cylindrical object, the pressure of said purge fluid in saidreservoir being greater than the static pressure of said fluidsurrounding said cylindrical object; and velocity detecting meansmounted in said purge fluid conduit means and detecting the variationsof velocity of said purge fluid in said purge fluid conduit means whichis indicative of the flow velocity of said fluid.
 2. A flow meter asclaimed in claim 1 wherein said purge fluid conduit means comprises apair of tubes which extend into said cylindrical object and are joinedtogether and connected to said reservoir through a common conduit andsaid velocity detecting means is mounted in one of said tubes.
 3. A flowmeter as claimed in claim 1 wherein openings in said cylindrical objectare a plurality of openings on each side thereof aligned in its axialdirection.
 4. A flow meter as claimed in claim 1 wherein said openingsin said cylindrical object are a pair of slits on each side thereof andextending in the longitudinal direction of said cylindrical object.
 5. Aflow meter as claimed in claim 1 in which said purge fluid conduit meanshave restrictors mounted therein.
 6. A flow meter as claimed in claim 1in which said purge fluid is air.
 7. A flow meter as claimed in claim 1in which said purge fluid is a liquid and the pressure of said liquid isapproximately equal to atmospheric pressure.
 8. A flow meter comprising:a cylindrical object to be immersed in fluid whose velocity is to bemeasured and producing Karman vortices in said fluid; a purge fluidreservoir; purge fluid in said reservoir; at least two openings formedin said cylindrical object on opposite sides thereof and said openingsformed to be substantially perpendicular to the flow direction of saidfluid; a pair of cavities formed in said cylindrical object on oppositesides thereof and said two openings communicating therewith; a partitionwall between said cavities; an aperture formed in said partition wall; apair of tubes connected to said reservoir and respectively communicatingwith said cavities; purge fluid supplied to said tubes which passesthrough said aperture and the pressure of said purge fluid in saidreservoir being greater than the static pressure of said fluidsurrounding said cylindrical object; and velocity detecting means formeasuring the flow of said purge fluid.
 9. A flow meter as claimed inclaim 8 wherein said velocity detecting means is mounted in one of saidtubes for detecting the variations of velocity of said purge fluid. 10.A flow meter as claimed in claim 8 wherein said velocity detecting meansis mounted in said aperture.
 11. A flow meter comprising: a cylindricalobject to be immersed in fluid whose velocity is to be measured and forproducing Karman vortices in said fluid; a purge fluid reservoir forsupplying purge fluid to purge said cylindrical object, the pressure ofthe purge fluid in said reservoir being greater than the static pressureof said fluid surrounding said cylindrical object; a pipe with one endconnected to said purge fluid reservoir and feeding said purge fluid outof an opening in said cylindrical object to purge at least one sidesurface of said cylindrical object and said opening formed to besubstantially perpendicular to the flow direction of said fluid; and afluid displacement detecting element mounted in said pipe means fordetecting the variations of velocity of said purge fluid.
 12. A flowmeter as claimed in claim 11 in which said cylindrical object hasprovided therein a plurality of axially aligned openings adjacent saidopenings on at least one side surface thereof and said plurality ofopenings formed to be substantially perpendicular to the flow directionof said fluid.
 13. A flow meter as claimed in claim 12 wherein saidopening in said cylindrical object is a slit extending in the axialdirection and formed to be substantially perpendicular to the flowdirection of said fluid.
 14. A flow meter as claimed in claim 11 inwhich said cylindrical object has formed therein two cavities, apartition wall between said cavities, an aperture formed in saidpartition wall, and said opening comprises a pair of groups of alignedopenings in the wall of said cylindrical object on opposite sidesthereof and formed to be substantially perpendicular to the flowdirection of said fluid, said group of openings of one sidecommunicating with the group of openings on the other side through saidcavities and said aperture and said pipe feeding purge fluid into one ofsaid openings, cavities or aperture so that it flows out of said groupsof aligned openings.
 15. A flow meter as claimed in claim 14 in whichsaid fluid displacement detecting element is mounted in said aperture.